1. Field of the Invention
This invention relates to optical fiber communication networks, and, more particularly, to method and apparatus using distributed Raman amplification and remote pumping for extending the reach and/or increasing the split ratio of passive optical networks.
2. Discussion of the Related Art
A passive optical network (PON) is a point-to-multipoint, fiber-to-the-premises, broadband network architecture in which unpowered optical splitters arc used to enable a single optical fiber to serve multiple customer premises. As shown in FIG. 1, a typical PON 10 includes an optical line terminal (OLT) 10.1 at the service provider's central office (CO) and a multiplicity of optical network units or terminals (ONUs or ONTs) 10.2 located at or in the vicinity of near end users; i.e., subscriber or customer premises (CPs; not shown). The OLT 10.1 is optically coupled to the ONU/ONTs via an optical distribution network (ODN) 10.4 comprising a transmission optical fiber 10.5 and a 1:N passive optical splitter 10.7 located within a remote node (RN) 10.6. The ODN is often referred to as the outside plant.
A PON is said to be passive even though it is apparent that the terminals (OLTs, ONUs and ONTs) of the network include active components and/or circuits that require electrical power. Nevertheless, as long as the ODN includes no components or circuits that require electrical power, it is common in the industry to refer to the entire PON as being passive.
The number of ONU/ONTs corresponds to the split ratio (1:N) of splitter 10.7. Each ONU/ONT 10.2 terminates the optical fiber transmission line and provides electrical signals over metallic lines to each CP.
A single fiber architecture between the OLT and splitter is made possible by using wavelength division multiplexing (WDM); e.g., in a GPON (gigabit PON) standardized with ITU-T G.984. The downstream signals are transmitted at a wavelength in the range of 1480-1500 nm and upstream signals are transmitted at a wavelength in the range of 1300-1320 nm. Continuous-mode downstream signals (e.g., 1490 nm signals from the OLT to the ONU/ONTs) are broadcast to each ONU/ONT 10.2 sharing the single fiber 10.5; that is, a downstream signal is split into a multiplicity of sub-signals that are directed onto separate optical fiber paths coupled to different ONU/ONTs. (The sub-signals at the output of the splitter are essentially identical to the downstream signal as received at the input of the RN but have lower power due to the inherent function of the splitter.) Encryption is used to prevent eavesdropping. On the other hand, burst-mode upstream signals (e.g., 1310 nm signals from the ONU/ONT 10.2 to the OLT 10.1) are combined using multiple access protocol, usually time division multiple access (TDMA). The OLTs control the transmission of the traffic from the individual ONU/ONTs onto the shared single fiber in order to provide time slot assignments for upstream communication.
PONs do not use electrically powered components to split the downstream signal. Instead, the signal is distributed among end users by means of passive optical splitters. Each splitter typically splits the signal from the transmission fiber 10.5 into N drop-line (or fan-out) fibers 10.9, where N depends on the manufacturer, and several splitters can be aggregated in a single remote node cabinet.
PON configurations reduce the amount of fiber and CO equipment needed compared with point-to-point architectures. In addition, a PON requires little maintenance and no electrical powering in the passive outside plant (ODN), thereby reducing expense for network operators. However, the maximum transmission distance (or reach) between the OLT and the farthest ONU/ONT, as well as the split ratio, are currently limited by various physical layer limitations and the PON protocol. For example, although the GPON standard (ITU-T G.984) allows for a logical reach of 60 km and maximum split ratio of 1:128, a 28 dB loss budget currently limits typical GPON deployments to a 1:32 split ratio and km reach. Of course, for a given loss budget, if a particular application allows for a smaller split ratio (e.g., 1:16), then the reach may be longer (e.g., 30 km). Conversely, if an application allows for a shorter reach (e.g., 10 km), then the split ratio may be larger (e.g., 1:64). However, some applications require both: a longer reach (e.g., 60 km) and a larger split ratio (e.g., 1:64).
There have been several reports of techniques to extend the reach of PON systems [e.g., K. Suzuki, et al., “Amplified gigabit PON systems”, J. Optical Networking, Vol. 6, No. 5, pp. 422-433 (2007); and P. P. lannone, et al., “Hybrid CWDM Amplifier Shared by Multiple TDM PONs”, Proceeding of OFC 2007, PDP-13 (2007), which are incorporated herein by reference]. In addition, GPON reach extenders have been standardized recently by the International Telecommunications Union. [See, ITU-T G.984.6.]
The reach extension approaches considered in G.984.6 require the use of electrically powered units in the outside plant containing well-known optical amplifiers or optical-to-electrical-to-optical (OEO) repeaters, but this design requirement negates some of the advantages of PON systems and may not always be practical or cost effective for network service providers/operators, particularly in certain environments where no electrical powering is available.
Thus, a need remains for techniques that extend the reach and/or increase the split ratio of PONs while maintaining a passive outside plant.
There is also a need for increasing the loss budget of PONs without requiring electrical powering in the distribution network of such a system.